1. Field of the Invention
The present invention relates to the field of fabrication of microstructures, and, more particularly, to a tool for chemically mechanically polishing (CMP) substrates, bearing, for instance, a plurality of dies for forming integrated circuits, wherein the tool is equipped with a conditioner system for conditioning the surface of a polishing pad of the tool.
2. Description of the Related Art
In microstructures such as integrated circuits, a large number of elements, such as transistors, capacitors and resistors, are fabricated on a single substrate by depositing semiconductive, conductive and insulating material layers and patterning those layers by photolithography and etch techniques. Frequently, the problem arises that the patterning of a subsequent material layer is adversely affected by a pronounced topography of the previously formed material layers. Moreover, the fabrication of microstructures often requires the removal of excess material of a previously deposited material layer. For example, individual circuit elements may be electrically connected by means of metal lines that are embedded in a dielectric, thereby forming what is usually referred to as a metallization layer. In modem integrated circuits, a plurality of such metallization layers is typically provided, wherein the layers are stacked on top of each other to maintain the required functionality. The repeated patterning of material layers, however, creates an increasingly non-planar surface topography, which may cause deterioration of subsequent patterning processes, especially for microstructures including features with minimum dimensions in the sub-micron range, as is the case for sophisticated integrated circuits.
It has thus turned out to be necessary to planarize the surface of the substrate between the formation of specific subsequent layers. A planar surface of the substrate is desirable for various reasons, one of them being the limited optical depth of the focus in photolithography, which is used to pattern the material layers of microstructures.
Chemical mechanical polishing (CMP) is an appropriate and widely used process to remove excess material and to achieve global planarization of a substrate. In the CMP process, a wafer is mounted on an appropriately formed carrier, a so-called polishing head, and the carrier is moved relative to a polishing pad while the wafer is in contact with the polishing pad. A slurry is supplied to the polishing pad during the CMP process and contains a chemical compound reacting with the material or materials of the layer to be planarized by, for example, converting into a reaction product that may be less stable and easier removed, while the reaction product, such as a metal oxide, is then mechanically removed with abrasives contained in the slurry and/or the polishing pad. To obtain a required removal rate while at the same time achieving a high degree of planarity of the layer, parameters and conditions of the CMP process must appropriately be chosen, thereby considering factors such as, construction of the polishing pad, type of slurry, pressure applied to the wafer while moving relative to the polishing pad, and the relative velocity between the wafer and the polishing pad. The removal rate further significantly depends on the temperature of the slurry, affected by the amount of friction created by the relative motion of the polishing pad and the wafer, the degree of saturation of the slurry with ablated particles and, in particular, the state of the polishing surface of the polishing pad.
Most polishing pads are formed of a cellular microstructure polymer material having numerous voids which are filled with slurry during operation. A densification of the slurry within the voids occurs due to the absorbed particles that have been removed from the substrate surface and accumulated in the slurry. As a consequence, the removal rate steadily decreases, thereby disadvantageously affecting the reliability of the planarizing process and thus reducing yield and reliability of the completed semiconductor devices.
To partly overcome this problem, typically a so-called pad conditioner is used that “reconditions” the polishing surface of the polishing pad. The pad conditioner includes a conditioning surface that may be comprised of a variety of materials, e.g., diamond that is embedded in a resistant material. In such cases, the exhausted surface of the pad is ablated and/or reworked by the relatively hard material of the pad conditioner once the removal rate is assessed to be too low. In other cases, as in sophisticated CMP apparatus, the pad conditioner is continuously in contact with the polishing pad while the substrate is polished.
In modern integrated circuits, process requirements concerning uniformity of the CMP process are very strict so that the state of the polishing pad has to be maintained as constant as possible over the entire area of a single substrate as well as for the processing of as many substrates as possible. Consequently, the pad conditioners are usually provided with a drive assembly and a control unit that allow the pad conditioner, that is at least a carrier including the conditioning surface, to be moved with respect to the polishing head and the polishing pad to rework the polishing pad substantially uniformly while avoiding interference with the movement of the polishing head. Therefore, one or more electric motors are typically provided in the conditioner drive assembly to rotate and/or sweep the conditioning surface suitably.
One problem with conventional CMP systems resides in the fact that consumables, such as the conditioning surface, the polishing pad, components of the polishing head, slurry batches and the like, have to be replaced on a regular basis. For instance, diamond-comprising conditioning surfaces may typically have lifetimes of less than 2,000 substrates, wherein the actual lifetime depends on various factors that make it very difficult to predict the appropriate time for replacement. Generally, replacing the consumables at an early stage significantly contributes to the cost of ownership and reduced tool availability, whereas a replacement in a very advanced stage of one or more of the consumables of a CMP system may jeopardize process stability. Moreover, the deterioration of the consumables renders it difficult to maintain process stability and to reliably predict an optimum time point for consumable replacement.
In view of the above-mentioned problems, there exists a need for an improved control strategy in CMP systems, wherein the behavior of consumables is taken into account.